the "make install" routine in different packages treats libexec files differently.
By default, they can be installed to:
..and probably others besides.
For consistency, I have been adjusting the ./configure switches, where necessary, so that libexec files are installed in /usr/local/lib/package_name.
Note that, since the libexec files are not called directly, the actually location is not that important, but cannot be changed after installation.